Traditional measurement approaches do not adequately capture the functional burden of coexisting diseases in older adults. To address this concern, we propose to develop and validate a novel approach based on the idea that functional burden can be characterized by patterns of walking activity measured over an extended period of time. One hundred older men and women representing a range of multiple medical conditions will be recruited to participate in a cross-sectional investigation. Walking activity will be recorded continuously over a two-week period using a highly accurate, step counting accelerometer. Activity will be characterized in terms of intensity (number of steps), frequency (number of activity-inactivity cycles), and duration (time spent walking at low, moderate, or high intensity.) Minute-to-minute fluctuations in the intensity of walking activity will be quantified in terms of variability (magnitude) and complexity (extent of randomness.) A variety of data reduction strategies will be explored. Secondary measures of function, disability, and self-efficacy will be collected to determine the validity of activity indices for capturing functional burden. The stability of activity indices will be determined over days and weeks. We hypothesize that greater numbers of morbidities will be associated not only with taking fewer steps, walking less often, and walking for shorter periods of time, but also with fluctuations in activity intensity that are less variable and complex. We believe that our extended monitoring approach will produce a physiologically-based characterization of functional burden that offers unique insight into the dynamics of interacting morbidities. The approach may contribute to the development of medical, surgical, or rehabilitative strategies for improving the ability of elderly patients to adapt to the physical demands of daily life. Study findings are likely to be relevant to aging studies arising from a variety of basic and clinical science disciplines.